1. Field of the Invention
This invention relates to improved recovery of hydrocarbons from a subterranean formation by a combustion process (fireflooding). In one aspect, the invention relates to an improved combustion process wherein the oil recovery rate is improved by increasing the pressure in the locus ahead of the combustion zone, as by throttling the production wells. In another aspect, the invention relates to an improved combustion process wherein the improvement involves a selective retarding of combustion front movement by throttling to increase produced gas pressure from at least one of a plurality of production wells which produce liquid hydrocarbons and gaseous products of combustion.
2. Brief Description of the Prior Art
A great need exists for increased production of hydrocarbons to meet increasing demands in the face of rapidly depleting reserves. One of the more promising approaches to solving this need involves enhanced recovery methods. Thermal recovery methods, in particular, in situ combustion methods, provide one means of recovering vast reserves of heavy petroleum deposits including tar sands and other reservoirs containing high viscosity materials which are not economically recoverable by other means.
U.S. Pat. Nos. 3,153,448; 3,208,519; 2,994,375; 3,171,479; 3,024,841 and 3,196,945 are exemplary disclosures of meritorious processes for the recovery of heavy hydrocarbons by thermal methods, in particular by in situ combustion methods (fireflooding).
Thus, it is known to recover hydrocarbons from a hydrocarbon-bearing subterranean formation, in particular, a heavy oil reservoir or tar sand, by penetrating the formation with a production well and an injection well, igniting the hydrocarbons in the deposit, injecting air to cause burning of a portion of the hydrocarbons in situ, and recovering hyrocarbons which are reduced in viscosity by the heat generated by the burning. Processes involving forward combustion wherein an oxygen-containing gas is injected into an injection well causing forward burning in the direction of a production well are known. Also known are reverse combustion processes wherein combustion is initiated in a production well with oxygen-containing gas injection from an injection well and movement of the firefront from the production to the injection well and production of hydrocarbons from the production well. It is also known to enhance the effectiveness of such fireflood processes by introduction of water into proximity with the burning zone.
Such combustion processes are disclosed to be particularly advantageously employed wherein the production well is the center well of a five-spot of nine-spot configuration when a forward combustion process is employed. Line drive configurations are also advantageously employed.
Advantageous and valuable though such processes are, certain problems are evident. Sweep efficiency of the front is often less than desirable because pressure and temperature are not high enough in the condensing steam zone preceeding the combustion front to fully mobilize the hydrocarbons in the formation. Also, because of the presence of reservoir irregularities such as high permeability streaks and/or fractures in the reservoir, the heat front may approach a producing well very rapidly in comparison to another producing well thus shortening the life of the recovery process and leaving substantial reserves in the reservoir. If the heatfront approaches a particular producing well more rapidly than the others, the well becomes hot early in the life of the project and presents considerable operating difficulties. Once the heatfront contacts such a production well, the well may also be lost. Our improvement invention provides a substantial advance in overcoming or mitigating such difficulties.